1. Field of the Invention
This invention relates to a machine tool for processing platelike workpieces, particularly for machining the contours of superimposed sheet metal elements, which machine tool comprises a work-supporting deck having a supporting surface and workpiece-fixing means, and a tool unit, which is mounted on a tool carriage and is movable over the supporting surface, wherein a holding-down device is provided, which is movable over the supporting surface in unison with the tool unit and comprises a holding-down shoe, which is adapted to be forced against the top surface of the workpiece to hold the workpiece in position close to the location at which the workpiece is being processed.
2. Description of the Prior Art
A workpiece cannot neatly be processed by such a machine tool unless the workpiece is reliably held in position, and in many cases it is particularly necessary to hold the workpiece firmly in position close to the location at which the workpiece is being processed. Particularly for a joint processing of superimposed workpieces, e.g., for a progressive cutting of blanks out of or for contour milling of sheet metal elements, it is necessary to properly hold the workpiece stack in position relative to the supporting surface and against a movement of the several sheets relative to each other. In the previous practice this has been accomplished in that stacks of workpieces were reveted to a bottom fixing pallet to form a work unit, which was then fixed to the work-supporting deck by existing fixing means, which may comprise holding-down claws or suction boxes for applying a vacuum to the supporting surface. In order to hold the workpiece in position also close to the location at which the workpiece is being processed, an additional holding-down device has been provided, which comprises an annular holding-down shoe, which may concentrically surround the tool spindle. During the processing of the workpieces that holding-down shoe is forced down against the stack of workpieces or the workpiece and is moved in unison with the tool so that the workpiece is held in position as desired near the location at which the workpiece is being processed. As is apparent from U.S. Pat. No. 3,587,391 or EP-A 0 010 043 the known holding-down devices are mounted directly on the tool unit so that they are compelled to follow the feed movement of the tool. As a result, it is difficult during a cyclic processing, e.g., during a progressive cutting of blanks out of sheet metal elements, to cut through the final land at the end of the processing because the severing of said land will separate the blank from the remainder of the workpiece and the feed movement of the tool and of the holding-down shoe may then result in a displacement of the parts which have been cut off so that the cut may not be as precise as is required. For this reason the parts to be cut out and the remaining parts must be riveted in spite of the provision of the holding-down means so that the known retaining means are expensive and require time-consuming operations as well as an inevitable perforation of the workpieces.